Improve default content

[sarahmonster]

I was just starting to file an issue to request MDX, and then I realised we'd already brought that over... but the file I was editing was a .md and so I assumed it wasn't there.

Let's maybe use the default starter content as a way to teach people (including myself) how to use the starter.

So, for instance:

• Show both Markdown and MDX in use, and be explicit with examples.
• Show more examples of different content!
• import { Button, Heading, Link, Paragraph } from '@octopusthink/nautilus' so I don't have to search + copy
• Show links to blog and portfolio (and maybe also the 404 page listing? see also Add a default page menu component #23) along with how to add, remove, or change content (Allow for easily disabling blog and portfolio #25)
• Link back to help docs if and when we have them, or at least GitHub!
• Provide some suggested starting edits: modify SiteSettings.js, add a Nautilus theme, add logos & icons, add a page, etc. (This should also go back to the docs.)